Split sheave continuously variable transmissions (CVT's) are used in a variety of recreational type off-road vehicles such as snowmobiles, golf carts, all-terrain vehicles (ATV's), and the like. CVT's, as their name implies, do not require shifting through a series of forward gears, but rather provide a continuously variable gear ratio that automatically adjusts as the vehicle speeds up or slows down, thus providing relatively easy operation for the rider.
Typically CVT's are comprised of a drive clutch, a driven clutch, and an endless V-belt disposed about the clutches. The drive clutch includes a pair of opposed sheaves, which together define a generally V-shaped "pulley" within which the V-belt rides. The driven clutch is similarly configured from a pair of opposed sheaves.
The sheaves of the drive clutch are normally biased (such as by a spring) away from each other and the sheaves of the driven clutch are normally biased toward each other. When the drive clutch is rotating at or near idle speed, the V-belt rides near the center of the drive clutch and near the outer periphery of the driven clutch. As the drive clutch rotates faster, a centrifugal mechanism in the drive clutch pushes one of the the drive clutch sheaves toward the other, squeezing the V-belt and pushing it radially outwardly. Because the V-belt is essentially inelastic, the belt is pulled radially inwardly on the driven clutch, causing one of the driven clutch sheaves to move axially away from the other driven clutch sheave.
Significant frictional forces occur between the clutch sheaves and the V-belt. Friction results not only from the inward and outward movement of the V-belt between the sheaves of the drive and driven clutches, but also from the constant compression of the clutch sheaves against the V-belt. That is, in order to enable the belt to transfer torque from the drive clutch to the driven clutch without slipping, it is important that the sheaves of the clutches squeeze relatively hard against the side walls of the V-belt.
Consequently, the friction of the V-belt against the sheaves creates a significant amount of heat. If this heat is not adequately removed from the sheaves, the belt can become quite hot, reducing its useful life.
Accordingly, it would be advantageous to provide for the efficient removal of heat from the clutch sheaves. For this purpose, some clutches are provided with vanes or ribs on the outer surface of one or more of the clutch sheaves. Such vanes or ribs create air turbulance that tends to assist in cooling the sheaves. Such turbulance is somewhat inefficient in removing heat, however, and it therefore reduces the performance of the vehicle more than would be desirable. Accordingly, it would be advantageous to provide for more efficient cooling of the clutch sheaves without significantly degrading the performance of the vehicle.